The present invention relates to compositions and methods for producing coatings on components used in harsh environment and to such components. More particularly, the present invention relates to compositions and methods for producing improved surface smoothness of coatings on components of turbine engines and to such components.
Metallic components of machinery must often be used in many harsh environments, being exposed to very high temperatures and/or to abrasive or corrosive gases. In such applications, these metallic components are typically coated with one or more layers of protective materials to extend their service life. In many applications, such as gas turbine engines, efficiency further dictates that the surfaces of the components exposed to the hot gas be as smooth as possible to minimize pressure drop due to surface roughness which results in friction loss. Components of gas turbine engines, such as compressor blades and vanes, are commonly coated with a corrosion-resistant coating. Such a coating provides cathodic protection of the stainless steel components from corrosion-causing elements in the inlet gas stream. To form such a coating, aluminum-rich particles or particles of a mixture of aluminum and other metals are mixed into a slurry that is applied to the surface of a turbine engine component. The coating is cured to solidify and harden the slurry matrix to form a durable coating. The coating is mechanically burnished, typically with a glass bead blast, to form an electrically conductive coating from a continuous network of the aluminum particles. As gas must flow past these turbine engine components, the surface coating must be as smooth as possible so not to compromise the aerodynamic efficiency of the gas turbine engine. Moreover, for economic and operational reasons, the protective coating should also be as thin as practicable. These desires have been satisfied largely, for example, by using nearly spherical particles having a small equivalent spherical diameter (xe2x80x9cESDxe2x80x9d) and a narrow particle distribution. As used herein, the term xe2x80x9cESDxe2x80x9d of a particle of irregular shape means the diameter of a sphere having the same volume as the particle. A xe2x80x9cmedian ESDxe2x80x9d or xe2x80x9cD50xe2x80x9d is the ESD for which fifty percent (by volume) of the population of the particles has ESD below that value. However, very small aluminum particles are pyrophoric, presenting handling difficulty, and a narrow particle distribution most often increases manufacturing cost.
Therefore, it is desirable to provide particles for coating compositions, which particles do not have the shortcomings of the prior art particles and can produce a coating having reduced surface roughness. In addition, it is also very desirable to provide a method for producing a coating having reduced surface roughness that uses such coating compositions. It is also very desirable to provide machinery components that have a reduced surface roughness and have extended service life in harsh environments.
The present invention provides a coating composition that comprises a mixture of at least a first type of particles and a second type of particles, which particles are dispersed in a liquid medium. The particles of the total population have a median ESD in the range from about 1 to about 7 microns, and the two types of particles have different median ESDs. The first type of particles has a maximum ESD of about 40 micrometers and preferably has a median ESD greater than about 5 micrometers. The second type of particles has a maximum ESD of about 20 micrometers and preferably has a median ESD smaller than about 3 micrometers.
The present invention also provides a method for producing at least a first coating on a substrate, which coating has a reduced surface roughness. The method comprises the steps of: (1) preparing a coating composition that comprises a mixture of at least a first type of particles and a second type of particles, which particles are dispersed in at least a first liquid medium; the two types of particles having different median ESDs; (2) applying the coating composition on the substrate; and (3) curing to solidify the coating composition to produce a coating having a reduced surface roughness.
In another aspect of the present invention, the method further comprises the step of densifying the coating.
In another aspect of the present invention, the method further comprises the steps of applying a second coating on the first coating and curing the second coating. The second coating comprises a third type of particles having a maximum ESD less than about 4 micrometers, which particles are dispersed in a second liquid medium.
The present invention also provides articles having protective coatings formed from coating compositions or methods disclosed above.